This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Others have applied tagging to SSFP imaging, and in these instances the applied tags required using additional RF pulses and interrupting the regular SSFP RF train using an approach to store magnetization along the longitudinal axis. Also, conventionally full echo read outs are used. Previously, the Common k-space approach to tagging in which horizontal and vertical tag patterns were applied during separate cardiac cycles was described. In this approach, it was noted that there was a SNR improvement in applying separate stripe patterns compared to applying two sets of stripe tags to produce a grid tag pattern. However, due to the full echo acquisition of each signal read out, Common k-space could not be applied to tags at 45° without extending the scan time to 200% of the conventional scan time. Thus as described, Common k-space was only applicable to acquire vertical and horizontal tag lines. One major advantage of acquiring tags at 45° compared to 90° is that in k-space corresponding “satellite” tag signals are about 30% (i.e. √2) closer to the central k-space line. The advantage of this is that a lower resolution k-space matrix can acquire proportionately more tag information in less time, producing superior tag definition compared to a corresponding 900 grid tag series. Also, in Common k-space, while it was noted that the central region of k-space need only be acquired during either the horizontal or during the vertical tag section, this region was acquired at reduced intensity compared to a non-tag scan. In the current invention—called PRESSTO (Partial Recalled Echo in Steady-State Tag Ordering), 45° tags are allowed, and the central region is acquired without any tags applied. Further, PRESSTO could be applied to acquired tags at 0° and 90°, as in common k-space, and in this case the major differences between the previously described common k-space and PRESSTO are: 1) PRESSTO additionally targets the middle region of k-space, which is acquired without any tags applied, 2) the Common k-space approach was applied to gradient echo imaging whereas PRESSTO is applied to SSFP imaging, 3) in Common k-space whole lines of k-space were acquired for each TR period, whereas in PRESSTO partial lines are acquired, 4) in Common k-space only one line of k-space was acquired per TR period and in PRESSTO two line segments are acquired per TR period.
The previously described FastHARP approach uses a multi-echo gradient echo approach (a multi-pass echo planar imaging approach) to acquired regions of k-space corresponding to specific tag information. In Fast-HARP, which is described in terms of gradient echo imaging, only one tag direction is produced per cycle, and only one region is targeted for acquisition, centered on the satellite region of k-space. A grid tag is produced by combining the two acquisitions, each acquired with one set of tag lines, with the each tag series being orthogonal to each other. In this respect FastHARP has similarities with Common k-space in that only one tag producing series is generated for each pass and it has a similarity with PRESSTO in that a partial region of k-space is targeted for each acquisition. The major differences between FastHARP and PRESSTO are: 1) Fast-HARP is described for a gradient echo signal whereas PRESSTO is applicable to SSFP signals, 2) FastHARP targets the major tag-defining satellite signal region of k-space whereas PRESSTO targets a larger region containing several signal satellites, 2) FastHARP does not acquire the central region of k-space whereas PRESSTO targets this region with a non-tagged acquisition.
Thus, while there are similarities with previously described approaches, no other approach realized the acquisition characteristics for PRESSTO. The Table summarises the similarities and differences between PRESSTO, Common k-space, and FastHARP.
PRESSTOFastHARPCommon k-spaceSteady state free precessionEPI imaging sequence,gradient recalled echoimaging sequencebased on gradient recalled echoimaging sequenceMultiple partial echo readMultiple partial echo readSingle full line echo readoutsoutsoutNew approach to tagConventional tag generationConventional tag generationgeneration*Specifically targets centerCenter of k-space notCenter of k-space acquiredof k-space without tagtargeted for acquisitionas by-product of taginformation*generationApproximately 90% ofApproximately 10% ofAll of k-space acquiredk-space acquiredk-space acquiredApplicable to 45° or 90°Applicable to 45° or 90°Applicable to 90° tagstagstagswithout time penalty, and to45° with a time penalty ofdoubling the scan timeSensitive to multipleSensitive to only the majorSensitive to multipletag-defining satellite signalstag-defining satellite signaltag-defining satellite signalsalong each major axisalong each major axisalong each major axis*indicates a unique feature of PRESSTO not used in any other technology. While PRESSTO has some overlap with FastHARP and Common k-space, no other approach has combined the features of PRESSTO in a SSFP tagging sequence, which as stated above, has fundamentally different conditions compared to GRE and EPI. The fundamental differences between SSFP, EPI and GRE place severe constraints on the imaging gradients in SSFP, which do not have to be met in the other two acquisitions. In SSFP, it is essential to keep the repetition time, TR, short (typically <4 ms) to avoid introducing significant artifact related to main field inhomogeneity effects. PRESSTO achieves the conditions required for SSFP imaging, while reducing the TR by at least 10%.
The main problem that the present invention solves is fading of grid tag lines as the cycle progresses in magnetic resonance imaging of the heart. In conventional approaches to tagging, the grid tag lines are applied at the ECG r wave, i.e. at the start of systolic contraction, and contrast of grid tag lines is generally acceptable throughout the systolic period. As the cardiac cycle progresses, the contrast-to-noise ratio (CNR) of the grid tag lines steadily deteriorates, such that during the diastolic period, the CNR of the grid tag lines is generally too poor to allow adequate tracking of diastolic recovery of the heart. The current invention addresses this problem in several ways: 1) In PRESSTO, Steady state free precession (SSFP) imaging is used instead of the more common gradient echo imaging. Since the CNR in SSFP imaging is superior compared to gradient echo imaging, the CNR is inherently higher. 2) Conventionally, one grid tag pattern is initially applied (e.g. at 45 degrees) and is immediately followed by a set of stripe tags oriented at an orthogonal angle (e.g. at −45 degrees). In this case, the two sets of stripe patterns destructively interfere with each other to further degrade the grid tag CNR. Further, by applying two sets of stripe tags in this manner, the grid tag pattern is applied over an extended time period (e.g. the time it takes to apply two stripe tag patterns may occupy 15 ms per stripe series, for a total time of 30 ms). In PRESSTO, at most one set of stripe tags is applied during any given cardiac cycle. This avoids the loss of CNR by destructive interference between stripe tags and reduces the grid tag application time to that of a single stripe tag application (e.g. about 5 ms in PRESSTO). 3) In conventional gradient echo based approaches, stripe tags are applied by a combination of multiple RF pulses (typically 3-5) with interspersed gradients. To apply this approach to SSFP imaging requires interrupting the SSFP sequence and temporarily “storing” the steady state signal while the tags are applied. This interruption to the steady state typically results in residual spurious signal that disrupt the following few lines of k-space, making this early data following application of the tag pattern un-useable. In PRESSTO, no disruption is required of the train of continuously applied RF pulses required to sustain the SSFP signal, and consequently no compensation or discarding of k-space data is required. Further, in PRESSTO, compared to the SSFP sequence, no additional RF pulses are required to apply the tag pattern; instead, a non-balanced gradient is applied. 4) In conventional grid tagging, typically, each lines of k-space is acquired in its entirety, which, due to the concentration of signal inherent in tagging, means that substantial regions of primarily noise data are acquired along with the smaller regions of useful signal data, resulting in an overall reduction of the CNR. In PRESSTO, in each acquisition only partial k-lines are acquired, targeted to where the primary data are expected, thereby further increasing the CNR. 5) In conventionally tagging, even if only a stripe tag pattern is acquired, the central region of k-space, which corresponds to the baseline image upon which tags are superimposed, suffers low CNR due to signal interference between this region and the tag pattern. In PRESSTO, the central region is specifically acquired without any tag pattern applied, and it is therefore acquired at the optimal CNR. 6) Alternative approaches that separately acquire two stripe tag data sets typically double the scan time compared to the standard grid tag data set. In PRESSTO, k-space is split into three regions, corresponding to two sets of orthogonal stripe tags and one central region without tags, and since each region is primarily targeted for acquisition using a partial k-space line signal readout, the scan time in PRESSTO is not extended beyond that of a single stripe tag acquisition scan.
All of these features of PRESSTO contribute to the superior CNR and overall short scan time, allowing it to be performed in a breathhold manner.